Communications systems using optical fibers have reached a state of great technical sophistication and are not only of great commercial importance at the present time but undoubtedly will continue to increase in importance. The most advanced systems at present have the capability of transmitting information at rates greater than 10 Gbit/sec over distances of hundreds of kilometers.
Present systems thus have tremendous capacity. However, further increases in capacity or configurations of transmitters, receivers, modulators, etc., that reduce the costs of systems or increase capacity are economically attractive. For example, early optical communications systems spanned long distances by using repeaters that periodically detected and regenerated the optical signal. Present day systems that span long distances use optical amplifiers which are cheaper than are repeaters. The commonly used optical amplifiers use a rare earth doped optical fiber. Erbium is widely used and erbium doped fiber amplifiers are frequently referred to by the acronym EDFA. The amplifiers use a laser as the source for the pump signals for the EDFA. The configuration or architecture of the optical communications system is also important with different configurations having different assets and liabilities. The star architecture is presently preferred for fiber amplifier systems because it can support a larger number of stations than can other architectures.
The star configuration may be either unidirectional or bidirectional. However, the star configuration suffers from a significant drawback; namely, a non-optically isolated EDFA produces amplified spontaneous emission(ASE) which degrades both the upstream and down stream sensitivities and may create system instabilities due to lasing caused by optical reflections. This drawback exists for both discrete and distributed gain elements.
There is an extensive literature that describes the prior art. See, for example, Bergmann, U.S. Pat. No. 5,140,655; Cassidy, Proceedings IEEE/LOS Optical Multi-Access Networks, paper PD2, July 1990; Goldstein, IEEE Photon. Technology Letters, pp. 390-393, April 1991; Perrier 4th Workshop on Optical Local Networks, Sep. 24-25, 1993, Versailles, France; Perrier, ECOC 92, pp. 289-292, Berlin, Germany, September 1992.
Other configurations, such as a ring or a Manhattan street architecture, are also of interest. It is desirable to have a distribution system in which a single channel may be received by many users.